This invention relates to fusion-type welding of members made of different, dissimilar alloys and, more particularly, to such welding of alloys at least one of which includes an element that can form a substantially continuous layer of a brittle, low ductility intermetallic compound in a resulting weld.
Power generating apparatus, for example gas turbine engines, includes, in the hotter operating sections such as the turbine section, components made of high temperature alloys based on at least one of the elements Fe, Co, and Ni. Sometimes called superalloys, a typical example of frequently used superalloys is the Ni-based superalloy that can included alloying elements for a variety of purposes including precipitation strengthening, solid solution strengthening, and/or high temperature environmental resistance. An example of one of such elements of particular interest to the present invention is Al included in Ni base superalloys at least for high temperature oxidation resistance and in some alloys for precipitation strengthening generally in amounts in the range of about 2–8 weight percent.
One type of turbine section component made of a Ni-base superalloy is an air cooled turbine blade described in U.S. Pat. No. 5,387,085—Thomas, Jr. et al. Generally, such a turbine blade includes at its radially outer end a radially outwardly extending rib, sometimes called a “squealer” tip, about the airfoil periphery. During service operation of such a turbine blade made of a Ni-base superalloy commercially identified as Rene' N5 alloy and more fully described in U.S. Pat. No. 5,173,255—Ross et al., various types of service damage were observed. In one example, it was observed that the radially outer blade end or tip portion had been damaged apparently by hot gasses flowing over or across the blade tip and/or interference between the rotating blade tip and surrounding stationary structure. This general type of operating damage has been observed for many years in the gas turbine engine art and a variety of methods and turbine blade tip configurations have been reported. For example, one is described in U.S. Pat. No. 4,169,020—Stalker et al. Another example of operating damage is article surface or coating damage or wear observed on discrete outer airfoil surfaces or areas along which higher temperature engine products of combustion flow. Examples of such surfaces or areas are described in U.S. Pat. No. 6,042,880—Rigney et al., and U.S. Pat. No. 6,106,231—Brainch et al.
When such damage occurs to a turbine blade tip or end portion and/or to an airfoil surface, at least based on economic considerations it is better to repair the damage than to replace the blade. It has been recognized that high temperature alloys based on at least one of the noble metal elements Ru, Rh, Pd, and Pt offer great potential for extending the temperature resistance and operating life capabilities of portions of turbine engine turbine blades subject to at least one of oxidation, sulfidation, and fluid flow wear resistance. Attachment of such an alloy to a specific, discrete, distinct location on an article, responsive to operating conditions, to repair damage or to enhance the capability of the article to resist operating damage, can provide such extended operating capability. Required is a joining method that can accomplish such attachment.
A variety of reported methods currently used in the repair of turbine engine components, including turbine blades, for joining or welding of members or surface portions result in melting and fusion at juxtaposed, cooperating interfaces between existing and replacement alloys. Such methods, well known in the art, include those commonly referred to as ordinary fusion, plasma, laser and SWET welding methods. Such methods involve applying heat energy at the juxtaposed interface surface portions for a time sufficient to result in melting and intermixing of elements of the alloys being joined. Such intermixing has been observed to result in creation of a broader liquidus—solidus band that upon cooling can generate stresses that can, under some circumstances, lead to cracking. Such circumstances upon subsequent cooling that result cracking in certain metal or alloy combinations include the formation of a layer or film of a compound that effectively is substantially continuous.
High temperature alloys such as Ni-base superalloys include at least one element, typically Al, which can form relatively lower ductility compounds, sometimes with other elements. In some Ni base superalloys, such compounds are generated as desirable, distinct island precipitates that tend to strengthen the alloy, for example, precipitates of the Ni3(Al,Ti) type. However, formation of a substantially continuous layer or film of a low ductility intermetallic at a weld between joined surfaces of certain dissimilar metals and/or alloys has been observed to result in weld cracking, either upon cooling after welding or after subsequent heating of the weld. Generally, such an intermetallic is referred to in the art as a brittle intermetallic compound.
One combination of such dissimilar metals and alloys that can result in formation of such a substantially continuous, detrimental layer at a welded junction is the fusion of a Ni base superalloy including Al with a metal or alloy based on at least one of the noble elements Ru, Rh, Pd, and Pt. For example, review of the phase diagram for Al—Pt shows a variety of intermetallic compounds of Al and Pt starting at about 65 wt. % Pt and extending up to about 95 wt. % Pt at a temperature up to about 1600° C. Accordingly, fusion welding of such combinations for a time in which Pt and Al can produce a substantially continuous layer of a brittle intermetallic compound in the weld region can result in a weld failure. Cracks in the weld or its heat affected zone can form during or after welding, or as a result of heat treatment during subsequent manufacturing operations, or heating and cooling during service operation. Therefore, current, typical fusion-type welding methods were found to be undesirable for welding a Ni-base superalloy, for example the above identified Rene' N5 alloy including Al, to a high temperature noble metal or metal alloy not including Al, for example an alloy including nominally, in weight %, about 25 Pd, 34.5 Pt, 0.5 Zr and about 40 Rh. Forms of such alloys are more fully described in U.S. Pat. No. 6,575,702 B2. Cracking at the interface weld occurred as a result of the formation of a substantially continuous layer at least one brittle intermetallic compound of Al.
Provision of a fusion type welding method for rapidly joining different, dissimilar metals and/or alloys, that otherwise can form the above described substantially continuous brittle intermetallic layer with certain metal combinations, would enable use of metals or alloys based on noble elements to enhance operating resistance to high temperature environmental conditions. For example, one such known fusion type welding method that has been reported is a percussion arc welding method used for rapidly joining different metals at a juncture under pressure through application of a brief burst of electrical energy. Such a method has been used in the microelectronics field for the joining of dissimilar metals. Application and specific control and selection of materials and parameters of such percussion welding method so that it inhibits the extent of such detrimental interaction between elements of high temperature and noble metals or alloys during welding to avoid formation of a substantially continuous layer of brittle intermetallic compound would enable their effective joining to enhance operating life of an article such as manufactured for high temperature use in a turbine engine.